How to make a Python operator as a Slider- Scripting an Addon
Set slider property to True

Consult the docs on UILayout.prop,


  prop(data, property, text="", text_ctxt="", translate=True,
  icon='NONE', expand=False, slider=False,toggle=False,
  icon_only=False, event=False, full_event=False, emboss=True, index=-1,
  icon_value=0)


To display the property in the UI  set it to True.  The default is slider = False


  slider (boolean, (optional)) – Use slider widget for numeric values


Sample code

def draw(self, context):
    self.layout(context.object, "prop", slider=True)


Make sure to set the minimum and maximum of the property, if unset the default values are +/- the maximum possible value on your system and as a result the slider will sit around the middle and be somewhat meaningless.

Tip: Right click and view source to see the code behind the UI.